/*
 * Copyright 2016 The Netty Project
 *
 * The Netty Project licenses this file to you under the Apache License,
 * version 2.0 (the "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at:
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations
 * under the License.
 */
package io.netty.resolver;

import io.netty.util.concurrent.EventExecutor;
import io.netty.util.concurrent.Future;
import io.netty.util.concurrent.FutureListener;
import io.netty.util.concurrent.Promise;
import io.netty.util.internal.PlatformDependent;
import io.netty.util.internal.UnstableApi;
import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.net.UnknownHostException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

/**
 * A {@link NameResolver} that resolves {@link InetAddress} and force Round Robin by choosing a
 * single address randomly in {@link #resolve(String)} and {@link #resolve(String, Promise)} if
 * multiple are returned by the {@link NameResolver}. Use {@link #asAddressResolver()} to create a
 * {@link InetSocketAddress} resolver
 */
@UnstableApi
public class RoundRobinInetAddressResolver extends InetNameResolver {

  private final NameResolver<InetAddress> nameResolver;

  /**
   * @param executor the {@link EventExecutor} which is used to notify the listeners of the {@link
   * Future} returned by {@link #resolve(String)}
   * @param nameResolver the {@link NameResolver} used for name resolution
   */
  public RoundRobinInetAddressResolver(EventExecutor executor,
      NameResolver<InetAddress> nameResolver) {
    super(executor);
    this.nameResolver = nameResolver;
  }

  @Override
  protected void doResolve(final String inetHost, final Promise<InetAddress> promise)
      throws Exception {
    // hijack the doResolve request, but do a doResolveAll request under the hood.
    // Note that InetSocketAddress.getHostName() will never incur a reverse lookup here,
    // because an unresolved address always has a host name.
    nameResolver.resolveAll(inetHost).addListener(new FutureListener<List<InetAddress>>() {
      @Override
      public void operationComplete(Future<List<InetAddress>> future) throws Exception {
        if (future.isSuccess()) {
          List<InetAddress> inetAddresses = future.getNow();
          int numAddresses = inetAddresses.size();
          if (numAddresses > 0) {
            // if there are multiple addresses: we shall pick one by one
            // to support the round robin distribution
            promise.setSuccess(inetAddresses.get(randomIndex(numAddresses)));
          } else {
            promise.setFailure(new UnknownHostException(inetHost));
          }
        } else {
          promise.setFailure(future.cause());
        }
      }
    });
  }

  @Override
  protected void doResolveAll(String inetHost, final Promise<List<InetAddress>> promise)
      throws Exception {
    nameResolver.resolveAll(inetHost).addListener(new FutureListener<List<InetAddress>>() {
      @Override
      public void operationComplete(Future<List<InetAddress>> future) throws Exception {
        if (future.isSuccess()) {
          List<InetAddress> inetAddresses = future.getNow();
          if (!inetAddresses.isEmpty()) {
            // create a copy to make sure that it's modifiable random access collection
            List<InetAddress> result = new ArrayList<InetAddress>(inetAddresses);
            // rotate by different distance each time to force round robin distribution
            Collections.rotate(result, randomIndex(inetAddresses.size()));
            promise.setSuccess(result);
          } else {
            promise.setSuccess(inetAddresses);
          }
        } else {
          promise.setFailure(future.cause());
        }
      }
    });
  }

  private static int randomIndex(int numAddresses) {
    return numAddresses == 1 ? 0 : PlatformDependent.threadLocalRandom().nextInt(numAddresses);
  }
}
